The present disclosure relates generally to turbofan engines and, more particularly, to turbofan engines having a variable area fan nozzle.
Aircraft noise pollution is a significant environmental problem for communities near airports. Jet engine exhaust accounts for a majority of the noise produced by engine-powered aircraft during takeoff. Because it occurs at a relatively low frequency, jet engine exhaust noise is not effectively damped by the atmosphere alone.
Bypass turbofan engines typically produce two exhaust stream components: the engine core flow and the fan flow. The engine core flow is discharged from a core flow nozzle after passing through a core engine. The fan flow passes through an annular passageway formed by a core engine nacelle which surrounds the core engine and a fan duct which surrounds at least a portion of the core engine nacelle. An outlet for the fan duct is defined intermediate the core nacelle and a variable area fan nozzle. In some implementations, the variable area fan nozzle is secured to the downstream end of an axially translatable thrust reverser sleeve, which forms a part of the fan duct. The fan flow exits this outlet. The engine and fan flows collectively produce thrust that propels the aircraft forward.
In bypass turbofan engines, the engine core flow throat area at the core flow nozzle and the fan flow throat area at the fan nozzle are preferably optimized for specific engine operating conditions. For example, during takeoff, a relatively high level of thrust is required from the engines as compared to lower levels of thrust that are required during cruise flight. Increasing the quantity or mass of airflow through the fan duct having a fixed throat area at the fan nozzle results in an increase in the velocity of the airflow. An increase in the nozzle exit velocity results in an increase in the amount of noise that is generated by the nozzle.
One approach to increasing the fan nozzle throat area as a means to reduce noise generated during high-thrust events such as during takeoff is through the use of movable flaps or petals which form the fan nozzle exit external boundary. The flaps or petals may be deflected outwardly to enlarge the throat area of the fan nozzle and thereby reduce the exhaust velocity or, conversely, they may be deflected inwardly to reduce the throat area of the fan nozzle and thereby increase the exhaust velocity. The flaps or panels may also be biased to one side or the other in order to provide thrust vectoring for increased maneuverability of the aircraft.
It is known to vary the area of the fan nozzle (thereby modulating the fan flow) by deflecting flaps or panels (hereinafter “petals”) attached to the trailing lip area of an axially translatable thrust reverser sleeve which exposes cascades that redirect the fan flow forward. As used herein (including in the claims), the term “thrust reverser sleeve” includes at least the following configurations: (1) a one-piece axially translatable sleeve that extends around a major portion of the circumference of the fan duct, from one side of the engine pylon to the other; and (2) two axially translatable half-cowls mounted on rails fixed to upper and lower beams and extending from the upper beam to the lower beam. In accordance with the latter configuration, the upper beam is the main hinge beam that allows the reverser to open for engine access and removal. The lower beam (referred to hereinafter as “latch beam”) provides a means for locking together the two half-cowls. Thus the second configuration typically has two upper hinge beams and two latch beams.
In accordance with above-described configurations, a lack of fan nozzle throat position control can lead to petal flutter. Also, leakage of high-pressure fan flow radially outward between the petals of the variable area fan nozzle can degrade engine performance. There exists a need in the art for improved variable area fan nozzle systems designed to control fan nozzle throat location and prevent air leakage radially outward through the fan nozzle.